1. The Field of the Invention
This invention pertains to building construction, and, more particularly, to novel methods and apparatus for anchoring building walls to foundations and lower floors thereof. The invention provides an automatic adjusting mechanism to remove slack in a hold down system caused by wood shrinkage over time or wood crushing caused by earthquakes.
2. The Background Art
Wood products change dimensions as moisture content changes. Floor systems using solid sawn joists typically shrink approximately five percent in dimensions across the grain. Under certain conditions they have been known to shrink six and one-half percent within a year. This shrinkage is typically part of the overall process and condition called xe2x80x9csettling.xe2x80x9d Settling actually includes both settling of foundations, as well as settling of walls due to shrinkage.
Testing and load rating has been completed for shear walls mounted to solid underlying surfaces. The solid surfaces are typically comprised of steel, concrete, or both. In tests wherein a wall is constructed, and immediately tested thereafter, test results are substantially better than those for walls that have existed over time. In a typical practice, a sill plate anchor or lower anchor is a threaded rod or an anchored strap capturing the base plate or sill plate of a wall (the bottom, horizontal member above which the studs extend vertically). Over time, ranging from several months to several years, wood loses moisture, shrinks, and the building settles. Threaded rod type anchors become loose. Strap type anchors buckle if positively engaged and become loaded in compression, or the like.
Current tiedown systems (including rods, straps, and the like) do not provide a solution for this problem. After a building xe2x80x9csettlesxe2x80x9d the wall can lift before it will re-engage the hold down structure before the tiedown is even loaded to begin resisting movement of the wall. Substantial building damage can result before the anchoring hardware is loaded (in tension). Hardware that does not immediately engage the base of an anchored wall can result in a 50 percent to 70 percent loss in lateral, load-bearing capacity.
The problem arises, typically, in wind storms of great power, or in earthquake conditions. A building under such circumstances may be violently loaded or shaken back and forth in a lateral direction with respect to the extent of the wall. If a shearwall is tightly restrained by its base to a foundation, loads may be smoothly transferred from a horizontal to a vertical direction. Loads are resolved in the foundation, where they appear as tension and compression forces.
Buildings are often composed of long walls, (walls with a length greater than the height) and short walls (walls that have a length shorter than the height). The uplift load on a particular wall is inversely proportional to the length of the wall. Tall narrow shear walls (as commonly found in nearly all homes) act as lever arms and tend to magnify the input load. In certain instances and depending upon wall structural configuration, the actual load on the anchoring system may be magnified to several times the original load. Gaps caused by wood shrinkage may further introduce an undesirable shock load to the anchoring system as the gaps are closed and the anchor system is finally loaded.
However, the as-built building is generally not the building that will be sustaining loads induced by earthquake shaking or by wind. Wood components of the building structure, including floors, sill plates, top plates, and studs, will shrink. Shrinkage varies greatly but it ranges typically from about one-quarter inch under the best of conditions, to well over one inch.
Moreover, under load, wood crushes or collapses in compression under the loading of a wall. Neither shrinkage nor crushing are well-accommodated or otherwise resolved in currently available systems. These problems lead to a significant reduction in the lateral, load-bearing capacity of shearwalls. Typically, based on testing, load-bearing capacity reductions range from about 30 percent to about 70 percent, depending on whether the rating used corresponds to building codes for property preservation, or life safety.
A better hold down or tiedown system including an improved take-up is needed to accommodate shrinkage of building materials. An improved tiedown system with such an improved take-up mechanism will improve the strength of shear walls subject to shrinkage of constituent materials.
In accordance with the above-identified needs, it is an objective to provide a self-powered take-up system for removing slack from between shearwalls and their anchors. It is an objective to provide a high reliability with a 30 year minimum life expectancy. It is desired that the apparatus be a non-reversing (locking in the extended position) design that may be shipped in a cocked, or preloaded position, ready to fire or deploy to extend in height, and having a trigger mechanism to actuate operation thereof. It is desirable to provide a simple mechanism that may be activated in the field with a minimum of skill and tools. In certain embodiments it is desirable to provide smooth and continuous motion of the take-up process, involving no stepped functions. Nevertheless, in other embodiments, step functions may be desirable. It is desired to minimize backlash to within thousandths of an inch.
It is desired to provide an apparatus that may be concentrically loaded, and thus able to tolerate and better center eccentric loads. In practice, few items are ever installed fully squared, and the take-up apparatus in accordance with the invention is preferably able to function at loads equal to the full tiedown design load for any anchor to which the take-up is attached. Ultimately, the tiedown should withstand the full ultimate load of a corresponding tiedown apparatus without jamming or deflecting substantially (e.g. more than one sixteenth of an inch).
In certain embodiments, the take-ups may be stackable to provide additional take-up capacity where more shrinkage, due to more buildup of wood, occurs. It is desired to provide standard sizes adapted to fit commonly used anchors and to fit within the space currently available for tiedown mechanisms. Thus, the take-up according to the instant invention may be sized for use with virtually any common size of tiedown system, or even for use in specially sized systems.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus and method are disclosed, in suitable detail to enable one of ordinary skill in the art to make and use the invention. In certain embodiments an apparatus and method in accordance with the present invention may include an extender of monotonically increasing length (or height) fitted between an anchor and a wooden structure for taking up the space caused by wood shrinkage or crushing.
One embodiment of an apparatus according to the present invention includes a take-up unit having a hole therethrough to accommodate an anchor bolt of a hold down system. The take-up unit is generally structured to be interposed in compression between a surface to be retained and a retention nut carried in threaded engagement by the anchor bolt. A key functionality of a take-up unit is the capability of automatically increasing in height. Such increase in height may be accomplished in discrete steps, or as a continuous and smooth function.
A take-up unit typically includes a base member and a sliding member. The base and sliding members are generally cooperatively structured so that relative sliding motion between the base and sliding members causes a change in height of the take-up unit. The base and sliding members also typically have an interface therebetween adapted to retain the members in close sliding relation. One such interface might be a mutually engaged thread interface between the base and the sliding members. A second such sliding interface between a base and sliding member is a dovetail joint.
An exemplary take-up unit also typically includes a biased element configured as an expansion producing element. One practical and highly preferred expansion element is embodied as a coil spring, the spring being interposed between the base and sliding members and preloaded to urge the members toward a position of increased unit height. Also preferably included is a trigger mechanism operable to maintain the unit in an installation configuration having a first height. The trigger mechanism is generally releasable to deploy the unit for an automatic increase in height to a second height subsequent to installation of the unit in a hold down system.
A take-up unit according to the present invention may also include a safety mechanism which may be provided by an interference between structure carried by the sliding and base members. The safety mechanism may be engaged, or enabled, during manufacture of the take-up unit subsequent to preloading one or more springs. One function of a safety mechanism is to prevent unintentional disassembly of the unit. A safety mechanism may determine a maximum range of relative motion between base and sliding members. An exemplary safety mechanism is formed by a flange carried by the sliding member, the flange being deformed into position to interfere with structure carried by the base member subsequent to assembly of the unit. The interference created between the flange and base member structure prevents unintentional disassembly of the unit thereafter. A snap ring may also be employed to perform the same function as the flange.
A sliding member may also include a cover to form a debris-shield over an advancing interface between the members. The advancing interface may be defined as including at least a portion of that surface which produces an additional increase in unit height, the advancing interface being in addition to any surface in contact between the members while in an installation configuration. A take-up unit may be structured and arranged for stacking a plurality of units inline in a hold down system to effect an increased range of total extension height.
A take-up unit may be fashioned to have a sliding member that includes a right-hand threaded hole for engagement with a commercial anchor bolt. The sliding member typically includes a bearing surface against which to receive a jam nut, the jam nut also being in engagement with the anchor bolt. To further prevent inadvertent rotation of the sliding member about the anchor bolt under the action of a preloaded spring, a thread having a different direction than the anchor bolt thread is preferably provided between the base and sliding members. To accommodate commercially available right hand threaded anchor bolts, a left-hand threaded engagement may preferably be provided as a slidable capture interface between sliding and base elements.